The terminal phase of myogenesis includes specific alignment or "recognition" of myoblasts, perhaps followed by specific cell-cell adhesion, but certainly followed by fusion to form the multinucleate myotubes. Cell surface proteins may play a significant role in, and perhaps provide all the specificity for, the critical pre-fusion events of cell association. I have proposed that each stage of myogenesis is characterized by a specific set of protien "receptors" on the cell surface which receive and transduce environmental signals, as well as participate in cell-cell recognition, alignment, and fusion. These "receptors" should appear and disappear in a time ordered sequence. We have already identified a set of developmentally regulated myoblast surface proteins which could be involved in these processes. This proposal delineates the further analysis of these proteins, including: (1) purification and production of antibodies to the regulated proteins; (2) use of the antibodies to define the biological role of the developmentally regulated peptides; (3) insertion of the purified peptides into biological membranes; and (4) in situ chemical characterization of the regulated proteins. A major thrust of this proposal is the use of non-fusing variants to analyze pre-fusion and fusion events. These variants, obtained without the use of mutagens, have been valuable in identifying a subset of the developmentally regulated proteins worthy of intensified study, and will be used extensively in the analysis of the biological role of the peptides. Temperature-sensitive non-fusing variants will be isolated and used to determine interdependence in the sequence of surface changes, as well as the time at which some of the critical events in myogenesis occur.